In facilities handling international cargo, such as harbors and airports, are reinforcing security check in regard to cargo for the purpose of safety. Recently, in developed countries, for example U.S.A., inspection for not only explosives but also special nuclear materials is being requested in advance.
Generally, X-ray inspection is used for the inspection for explosives and nuclear materials. However, the X-ray inspection method is technically limited. Accordingly, an inspection method using a neutron source is being raised as a method for detecting the nuclear materials, U 235 and Pu-239, included in the cargo.
As a neutron source for detecting the nuclear materials, a Cf-252 isotope is generally used due to its convenience in use. However, a half-life of Cf-252 isotope is approximately 2.8 years, that is, a neutron production rate reduces so fast that the neutron generation rate may reduce to almost 1/10 or less after about eight years of use. In addition, handling of the Cf-252 isotope is not easy because a dedicated container is necessary when storing or transporting the Cf-252 isotope. Accordingly, when the nuclear material inspection method using the Cf-252 isotope is used, a charge cost of the Cf-252 isotope may be increased year after year due to the short half-life. In addition, a neutron generator is relatively uneconomical since an initial equipment of the neutron generator is expensive.